What is Nick's socio-economic background?
Nick hails from wealth. While he is not exuding in wealth, as his living condition at the start of the novel, he does come from a well- to - do background. His relationship to Daisy allows him to be able to move freely in the upper echelons of wealth, and allows him to procure a relationship with Jordan. The opening lines of the novel about his father telling him to reserve judgment because "not everyone has had the same benefits as you" might also indicate that Nick is not from a poor or economically deprived background. He is designed to be the type of figure that serves as the moral center or compass of the work, so while his background economically is important, it does not necessarily define him as it does Tom, Daisy, or Jordan.
You can find answers to this in Chapter 1.
I guess that you would probably say that Nick is from the upper class. That depends a bit on how you define the upper class, but he would most likely qualify.
We know that he is from a family that is pretty rich. He says that the family has been "prominent" and "well to do" for three generations in some city in the Midwest. We know also that he went to Yale, which in those days was a college that you only went to if you were pretty well-off.
However, Nick is not so rich as to be able to afford a really big house while working in New York, so he must not be filthy rich.
Nick is from the Midwest and has family that is well off but he is seeking to become his own man, he wants to be a bond man in New York.
My family have been prominent well-to-do people in this middle western city for three generations. the Carraways are something of a clan, and we have a tradition that we're descended from the Dukes of Buucleuch.
He goes on to tell us the real truth is that his grandfather started a hardware business that Nick's father still runs today.
Nick could be classified with that description as middle-upper class.